1. Field of the Invention
This invention relates to improvements in transistor circuits, and, more particularly, to improvements in circuits for reducing the turnoff time of power transistors, and still more particularly, to improvements in circuitry for reducing the turnoff time of bipolar transistors used for driving inductive loads, field coils of a dc motor, or the like.
2. Description of the Relevant Art
The problem addressed by this invention is encountered especially in integrated power drivers when used as current sources. Typically, especially in integrated processes, the transistor types used for power handling are either bipolar NPNs or n-channel MOS. When driving an inductive load, or in general, a load which may have any inductive (parasitic) component, a recirculation or freewheeling diode may be provided to supply a path for the load current at turn off.
Thus, with reference to FIG. 1, one prior art circuit embodiment 10 typically may include an NPN transistor 11 that provides drive current to an inductive load 12 and other load circuitry, indicated generally as a box 13. A diode 15 is connected in parallel with the inductive load 12 and other load components 13 to provide a recirculation path for the load current in the coil 12 when the transistor 11 is turned off. Thus, as is known, when the current to the inductive load 12 is turned off, the inductive action of the coil 12 will tend to maintain the current flowing from the emitter connection through the coil 12 and load 13 to ground. This continued current is enabled by the provision of the diode 15.
In a single supply application, turning off the power device may, in fact, be difficult, due to the fact that the load pulls the emitter (or source of an FET) below ground. This renders useless the connection of the power device's base (or gate of an FET) to ground. With reference now to FIG. 2, a circuit similar to the prior art circuit of FIG. 1 is shown, the corresponding parts being denoted by a prime ('). A second NPN transistor 18 is provided with its collector emitter path connected between the base of the NPN transistor 11' and ground. A signal input is provided on the base of the NPN transistor 18 to receive a low-to-high transition signal 20 simultaneously with the high-to-low transitioning signal 21 applied to the base of the NPN transistor 11'. Since the emitter of the NPN transistor 11' is pulled below ground, even the provision of a second NPN transistor 18 to attempt to turn off the first NPN transistor 11' renders operation of the circuit of FIG. 2 undesirably inefficient.
As shown in FIG. 3, the typical solution is the connection of a transistor across the base-emitter (or gate-source of an FET) terminals of the power device, thus shorting them to turn the power device off. The circuit embodiment 10" of FIG. 3 is constructed in a similar fashion to that of the embodiments 10 and 10' of respective FIGS. 1 and 2. The corresponding components of the circuit embodiment 10' are denoted in FIG. 3 by a double prime ("). In the circuit embodiment 10", a second NPN transistor 25 is provided. The collector emitter path of the transistor 25 is connected between the base and emitter of the NPN driver transistor 11". The base of the second NPN transistor 25 receives a control signal. The control signals to the respective bases of NPN transistors 11" and 25 are shown, with the high-to-low transition control pulse 30 that is applied to the base of the NPN transistor 11' falling simultaneously with the change in state from low to high of the control pulse 31 applied to the base of the NPN transistor 25.
Transistor 25, therefore, must be turned on at the instant the biasing of transistor 11" is removed. Unfortunately, particularly in the case of bipolar power transistors, removing the charge stored in its equivalent input capacitance requires the transistor 25 to quickly reach a high conductance state, which in turn requires its input equivalent capacitance to be charged immediately. This is difficult to achieve without expending a lot of current in the circuit that drives the transistor 25.